The present invention relates to a multi-channel power supply selector for selecting one of a plurality of power supplies for use in a high availability multi-redundancy power supply system.
FIG. 1 schematically illustrates a prior art arrangement in which two power supplies A and B, are connected to an output node O via a xe2x80x9cdiodexe2x80x94ORxe2x80x9d arrangement comprising first and second diodes 2 and 4. In the arrangement shown, which is typically for use in the telecommunications industry, each of the supplies A and B has a nominal output voltage of xe2x88x9248 volts in the USA and xe2x88x9260 volts in Europe, and hence the diode OR arrangement selects the most negative of the supplies. A hotswap device 6 is also provided in order to control the board inrush current that occurs when a newly swapped board is first connected to the power supply. The inrush current results from the rapid charging of previously discharged capacitors on the board. The hotswap control 6 performs a current limiting function in order to prevent an excessive inrush current from occurring. Such in-rush currents can cause damage to components and circuit board tracks in the current path and may also give rise to voltage spikes on the power supplies and which might be communicated to other equipment in the telecommunications system (not shown) In a known improvement of the prior art system, each of the diodes 2 and 4 is replaced by a synchronous rectifier, ie a MOSFET transistor, and a control circuit such that the MOSFET for the most negative supply is switched fully on and the MOSFET for the other supply is switched fully off. Thus, the functionality of the diodes is completely replicated, but without incurring the penalty of a diode voltage drop across a conducting diode.
According to a first aspect of the present invention there is provided a multi-channel power supply selector for selecting one of a plurality of power supplies for connection to a load, comprising of at least one controller and the plurality of channels; each channel comprising a first device responsive to the controller and operated as a switching device for selecting a power supply connected to said channel in series with a second device operating as a current limiting device or a further switching device responsive to the controller.
It is thus possible to provide a selection system in which a plurality of identical channels are provided, each comprising a first device functioning as a selection device and a second device as a switching and/or current limiting device. This has the advantage that, should one of the power supplies become too negative to be usable, then even if the first device becomes unable to disconnect the power supply due to its parasitic sourcexe2x80x94drain diode, the second device in that channel can be operated in order to cause an effective disconnection of the power supply from the load. This contrasts with the prior art arrangement shown in FIG. 1 where if a fault should occur on power supply A such that it becomes highly negative in voltage but out of the safe operating range whilst simultaneously B is still acting as a good supply, then supply A will be selected in preference to B thereby resulting in potential damage to the equipment connected at the output node O or unnecessary tripping of any subsequent over-voltage detection circuitry.
Advantageously the first devices are semiconductor devices. The use of semiconductor devices, such as transistors, gives rise to a much faster switching action than could be achieved with electromechanical devices. Advantageously the switching devices are field effect transistors, FETs. Field effect transistors can, in general, be switched hard on giving rise to low voltage drops VDS ON as a result of their low effective on resistance RDS ON.
Advantageously the second devices, which serve as current limiting devices are also field effect transistors.
In terms of field effect transistor technology, MOSFET transistors are currently preferred, and this term encompasses subgroups of MOSFET construction such as VMOS.
MOSFET transistors include an internal source drain diode which can be regarded as a parasitic diode resulting from conduction from the source to the drain via the transistor substrate and who""s presence is a consequence of the way these devices are fabricated. This has the consequence that if the drain becomes more negative than the source then there is a current path from the source to the drain irrespective of the voltage applied to the transistor""s gate. This is because even though the transistor itself is off, its parasitic diode has become conducting. In the present invention this problem is overcome as in each channel first and second MOSFET transistors are provided in back-to-back configuration. In the preferred embodiment of the invention the source of one transistor connected to the source of the other transistor. This ensures that irrespective of the voltages occurring at the load or the power supply, the internal source drain diode in one of the transistors will be reversed biased and hence no current path exists between the load and the supply. Given that the power supplies associated with telecommunications equipment are nominally rated at xe2x88x9248 volts but may produce voltages as low as xe2x88x9280 volts, then it might be assumed that the power supply selection controller would have to be fabricated, for example as an integrated circuit, in order to withstand at least 80 volts across the chip. In fact the energy stored in on-card inductances can give rise to large voltage spikes as the card is removed and therefore fabrication technologies capable of surviving transient voltages in excess of 150 or 200 volts are required. However, the inventors have realised that with a modification of the circuit topology the voltage appearing across the controller can be much reduced. This has the advantage that dissipation within the device will also be reduced and that also lower voltage fabrication technologies can be used. In particular where the power supply selection controller is formed from a plurality of sub-controllers each controller has one power supply connection to ground via a voltage dropping element, such as a resistor, such that the voltage appearing across the controller itself is significantly reduced. Furthermore, by having a second power supply connection to a node positioned intermediate the series connected transistors it is possible to ensure that the controller can always produce a gate voltage equal to the lower one of the source voltages of the associated transistors (via the parasitic source drain diodes) thereby being in a position to switch them off.
Advantageously each channel is provided with its own sub-controller, and the controllers communicate with one another in order to form a combined control system. Advantageously the controllers communicate via a differential current communications link such that the absolute voltage of one controller with respect to the other call float.
Preferably one of the controllers functions as a master controller and is provided with a representation of the voltage of each of the power supplies. The voltage may be divided via potential divider in order to protect the controller from having to see large voltages. The pickup point for the voltages does not have to be at the output of the respective power supplies, but can in fact be at the node between the first and second serially connected transistors in each one of the power supply channels. A connection at this point works since, if a power supply becomes more negative than the other power supply the parasitic diode within the MOSFET directly connected to the power supply conducts in forward bias therefore causing the power supply voltage to be xe2x80x9cvisiblexe2x80x9d to the selection circuit.
Advantageously each controller is arranged such that, in the event that the power supply it is monitoring is disconnected or fails, the controller can still receive power via current flowing from the output node back through the second transistor""s source-drain diode to the supply connection of the controller. Furthermore, placing the supply connection at the junction between the first and second serially connected field effect transistors, rather than at the output node, has the advantage that the internal voltage seen by the controller is not perturbed in the event that the controller has to operate the second transistor in order to perform a current limiting in order to maintain an inrush current within an acceptable range. Also, even if the power supply in the channel monitored by the master controller becomes faulty, the master controller itself will still receive a supply in order that it can perform a master/slave comparison and instruct the slave to take over if it can.
According to a second aspect of the present invention, there is provided a power supply selection control system, comprising a first controller having first and second outputs for controlling first and second serially connected field effect transistors disposed between a first power supply and an output node; a second controller having first and second outputs for controlling first and second serially connected field effect transistors disposed between a second power supply and the output node, and wherein in each channel the field effect transistors are connected such that the internal source-rain diodes thereof are connected in mutual opposition.
It is thus possible to provide a power supply selection system in which disconnection of a power supply is ensured irrespective of the voltage it outputs, whether in a functioning or a fault mode.